Actuators with variable stiffness and methods for controlling of their stiffness are of industrial interest and have an increasing potential of industrial applications.
In particular, such actuators may be utilized in industrial robots for motion and control of the actuated links of the robot.
The problem of adaptation and control of the stiffness and not only of position of the links of the robot during its motion arises when a robot is expected to perform motion tasks involving or in the presence of humans, or when collisions with the environment are possible.
In these cases, a control of the motion should provide a desired accuracy of velocity and position accuracy, which may be different in different stages of motion of the robot, while, at the same time, minimizing the risk that the robot and the environment are damaged and humans in the working envelope of the robot are injured due to collisions.
Several solutions have been the studied to decouple part of the inertia of the links of a robot from the end-effector link in order to reduce the kinetic energy transferred during a collision.
One of the first solutions that was attempted was to cover the links of the robot with soft panels, such as pads, in combination with collision sensors, in such a way that, when a collision happens, the soft panels deform giving time to the sensors to detect the collision and to the control system to react to the collision, for example by stopping the motors or inverting their directions of rotation.
Another solution that was considered was to mount passive compliant elastic elements between each motor and the link it drives. These elastic elements limit mechanically the inertial torque that each motor can transmit between the preceding and following link in the event of collision.
With the use of passive compliant elements in series with the motors, it is not possible to adapt the stiffness to the motion task requirements and, consequently, either the robot is in a precise position but is stiff or has a coarser accuracy of position but is better compliant.
Like in human and in animal beings, what is researched is the adaptation of the motion accuracy of the robot and of its stiffness according to the motion to be accomplished and the task of the robot.
If, for example, the task consists of moving an object from location A to location B, geometric accuracy during the trajectory from A to B can be coarse since the requirement is only to move the object from A to B, independently of the velocity with which the task is performed.
In a human, such a repositioning operation is performed with the muscles of the arm i a relatively relaxed state, just supporting the load with no tensions in antagonistic muscles to stiffen the arm. This way, the arm operates at low stiffness.
Differently, if a precise motion must be performed, such as during the assembly of two small parts or the threading a needle, high accuracy and low velocity are generally required.
To perform such movements, a human being increases the stiffness of all muscles involved in the motion, agonistic and antagonistic, in order to increase position accuracy at every point of the trajectory.
It may happen that the external force applied by the arm to the environment during the performance of the task is the same in the two cases of coarse and accurate motion tasks, but, in the case of an accurate motion task, the absolute values of the forces in the arm are higher although the difference is the same at same external force.
The above considerations have suggested the development and use, in robots, of systems for an active adaptation of the stiffness of the joints to the requirements of motion and task, such as variable stiffness actuators.
Several variable stiffness actuators are discussed in the prior art. Such actuators use mechanical springs and other elastic elements together with motors that command the positions of the links of the robot.
Each link of the robot mounts a motor that commands the position of the link, and the stiffness is adapted on the base of sensory feedbacks.
The effectiveness of this approach is limited by the limited bandwidth of the system, which is due to the delay of response of the sensor and the time of detection, transmission and use of the information before the motor is consequently operated.
Today, three families of architectures of variable stiffness actuator have been introduced: Serial-type Dual Actuator (SDA), Parallel-type Dual Actuator (PDA), and Hybrid-type Dual Actuator (HDA).
The SDA use, for each axis, a main actuator used to command the position and velocity of the driven link, and a secondary actuator responsible for the variation of the stiffness.
The PDA use a principle similar to the one in a human arm, with an agonistic and an antagonistic muscle. Two actuators operate in parallel the driven link, and a nonlinear elastic element is mounted in series to each actuator rendering independent the control of position and stiffness.
The HDA use two actuators arranged in any combination different from the serial one used in the SDA and the purely parallel one used in the PDA. For example, the two actuators may apply to the driven link a variable force at a variable distance from the axis of rotation with a nonlinear elastic element present at the point of application of the force.
All these types of actuators involve mechanical and electronic components which are subject to wear and failure that compromise the functioning of the system, for example, elastic elements like mechanical springs.
Moreover, due to the complexity of the design, these actuators can hardly be used at a micro scale, in applications that require small dimensions.
Therefore, there is an unsatisfied practical need for a device that overcomes the described above limitations and disadvantages, which are typical of the variable stiffness actuators in use. Such device should use simple and cost-effective solutions, and should be easy to integrate in robots whose tasks comprise interactions with humans.